This invention relates to a motor driver for driving a three-phase brushless motor.
Sensorless motor drivers not using a sensor such as a Hall element for phase detection are used as a motor driver for driving a brushless motor for rotating the cylinder of a video tape recorder or the spindle of a floppy disk drive. Conventional sensorless motor drivers are switched on and off such that the currents passing through the coils of the brushless motor will each have a rectangular waveform with phase differences of 120xc2x0 as shown in FIG. 1, and the angular position (phase) of the rotor of the motor is detected during its off time (of phase period of 60xc2x0). Motor drivers which are switched on and off in this manner have the problem of generating noise in the motor while the motor is rotating. Since the current passing through the motor changes suddenly with a mode of switching of this kind, noise is generated by the induction voltages generated in coils (hereinafter referred to simply as the noise) whenever the on-off condition of a current changes (or at moments when the polarity of a passing current changes) as shown in FIG. 2. Since such noise is superposed onto detection signals for the motor position, the rotation of the motor becomes unstable.
In view of problems of this kind, motor drivers for passing currents through coils over 150xc2x0 in phase with sloped current waveform have been considered. With this mode of operation, as shown in FIG. 3 where the mode of operation by passing current over 120xc2x0 in phase is shown by broken lines, currents are passed smoothly over 150xc2x0 in phase and the angular position of the rotor of the brushless motor is detected during periods of 30xc2x0 in phase. Since the waveform by this mode of operation is sloped and the currents through the coils change gradually, the noise of the motor during its rotation can be made smaller, but there is a limit to how much the noise can be reduced and, as shown in FIG. 4, there still is substantial noise even by this mode of operation with currents passed over 150xc2x0 in phase while the brushless motor rotates. Noise is smaller in this case than if the current is passed over 120xc2x0 but the situation is still the same in that the rotation of the motor is made unstable by the noise.
In order to address this problem of noise, it has been known to generate a so-called noise mask signal in order to prevent the noise of the coils from passing through. Since noise is generated when the on-off condition of the currents is changed, or at moments when the polarity of a current is reversed, noise mask signals are generated at this timing, as shown in FIG. 5, such that the noise will not be transmitted to the drive signal synthesizing circuit for controlling the currents through the individual coils during the periods (herein referred to as the noise mask periods) while a noise mask signal is being generated. Since the timing of the generation of noise can be determined by the signals outputted from this drive signal synthesizing circuit, the mask signal generating circuit charges and discharges a capacitor, uses a comparator to detect its charging and discharging voltages and makes use of the time required for the capacitor to charge and discharge to thereby determine the noise mask period for the noise mask signal.
Let us assume that the charge-discharge voltage of the capacitor, adapted to charge and discharge as the rotor rotates, varies as shown in FIG. 6 (by Graph (a)) with a triangular waveform. The comparator compares it with a standard voltage VS at a constant level and a low-level mask signal is generated as shown in FIG. 6 (by Graph (b)) during the periods in which the charge-discharge voltage is higher than the standard voltage VS. Thus, if the speed of rotation of the motor is increased, the noise mask time becomes accordingly shorter but the detection time for the angular position between two successive noise mask signals is constant, independent of the speed of rotation of the motor. Methods of this kind have been disclosed, for example, in Japanese Patent Publication Tokkai 5-344782, 6-311784 and 8-33382.
If the speed of rotation of the motor is increased and the period of the cycle of the triangular voltage waveform becomes short, as shown in FIG. 6 (by Graph (c)), however, the noise mask time becomes correspondingly short as shown in FIG. 6 (by Graph (d)) and hence it becomes impossible to remove the noise. As a result, the rotary motion of the motor becomes unstable.
This problem can be solved if the standard voltage Vs for comparing with the charge-discharge voltage is made smaller such that the noise mask time can be sufficiently long. If the noise mask time is made longer, however, the detection time for detecting the angular position of the rotor of the brushless motor becomes shorter. As a result, it may become impossible to detect the angular position of the rotor and to drive the brushless motor properly.
It is therefore an object of this invention, in view of the problems of the prior art technology described above, to provide a motor driver capable of gradually adjusting the noise mask time and the time for detecting according to the speed of rotation of a brushless motor.
A motor driver according to an embodiment of this invention for driving a three-phase motor may be characterized as comprising a synchronized signal generating means for generating signals which are synchronized with the rotation of the motor by comparing each of induction voltages generated in the coils of the motor with a specified standard voltage, a triangular wave signal generating means for generating a triangular wave signal, the frequency of the triangular wave signal increasing as the speed of rotation of the motor increases, a noise mask signal generating means for varying a standard value according to a change in the amplitude of the triangular wave signal and generating a noise mask signal by comparing this standard value with the triangular wave signal, a noise mask means for removing noise from the synchronized signals by using the noise mask signal, and a current supplying means for supplying currents to the coils of the motor by using the synchronized signals from which noise has been removed by the noise mask means. With a motor driver thus structured, noise can be effectively eliminated because both the noise mask time and the position detecting time can be varied gently. The rotation of the motor can also be stabilized because the noise mask time can be prevented from becoming short abruptly even if the speed of rotation of the motor is increased. According to one embodiment of the invention, the standard value which is used by the noise mask generating means is maintained to be at a constant ratio with respect to the amplitude of the triangular wave signal and hence can be obtained by a simple calculation and increased or decreased according to the amplitude of the triangular wave signal.
According to another embodiment of the invention, the induction voltages in the three coils of the motor are compared with a single specified standard voltage. This serves to eliminate the variations among the three phases of the synchronized signals and stabilize the rotation of the motor.
According to still another embodiment of the invention, the currents to be supplied to the coils are arranged not only to have a sloped portion but also such that the slope of this sloped portion will decrease as the rotation of the motor becomes slower. Even with such a sloped portion provided in order to vary the coil current gently, if the slope of this sloped portion is constant, independent of the speed of rotation of the motor, the slope of the sloped portion becomes relatively sharp as the speed of rotation of the motor decreases and the waveform approaches a rectangle. According to this embodiment of the invention, the slope of the sloped portion is caused to gentler as the speed of rotation of the motor becomes low such that noise can be eliminated even when the motor is rotating slowly.
Still another motor driver embodying this invention may be characterized as being different from the motor driver described above wherein the triangular wave signal is generated such that not only its frequency but the rate of its increase becomes larger as the speed of rotation of the motor is increased and that the triangular wave signal thus generated is compared with a specified standard value to generate the noise mask signal. A motor driver thus structured is also capable of changing both the noise mask time and the position detection time gently according to the speed of rotation of the motor. Even if the speed of rotation of the motor increases, the noise mask time can be prevented from becoming short abruptly.
Many of the inventive elements described above can be combined in a single embodiment.